In commercial mining applications blast holes are drilled, loaded with bulk explosive and the bulk explosive initiated. This is typically done using a so-called booster. This is a separate, relatively small explosive charge that is housed in a shell that is designed to receive a detonator. The detonator typically takes the form of a cylindrical cartridge and includes a base charge at one end. A lead (for signal transmission to fire the detonator) extends from the other end of the detonator. In use, a detonator is inserted into the booster, the booster is positioned in a blast hole and surrounded by bulk explosive. The detonator is then fired thereby triggering detonation of the explosive charge of the booster. In turn, that causes detonation of the bulk explosive.
Manufacture of a booster typically involves casting a molten explosive composition (usually Pentolite) in a suitably designed shell. The explosive composition is typically cast (poured) around metal (e.g. brass) pins suitably positioned within the cavity defined by the booster shell. After the explosive composition has solidified these pins are removed to provide tunnels (passages) that are adapted to receive a detonator. These cast boosters typically have at least two such detonator tunnels extending through the cast composition to allow a detonator to be fed fully down through one tunnel and return up through the other which will have a blind end or stepped end which functions as a stop position for the end of the detonator. The detonator lead (extending out of the top of the booster) is then pulled taut and the booster with detonator (primed booster) is ready to be positioned in a blast hole.
A problem that has been observed with this form of booster design is that as the cast explosive cools and solidifies it shrinks (the shrinkage rate is approximately 7 volume %) and this results in the composition developing shrinkage voids at its upper end, i.e. at the top of the booster. These shrinkage voids can lead to unreliable initiation of the booster because, when loaded in the booster, the detonator is oriented such that the base charge of the detonator is located towards the top of the booster and thus in proximity to any shrinkage voids that will be present. The presence of the voids tend to impair communication of energy from the base charge of the detonator to the cast explosive in the booster, thereby leading to unreliable initiation of the booster.
This problem can be mitigated by minimising the amount of voids present in the cast explosive composition, for example, by casting the explosive composition in stages with at least partial cooling of the composition being allowed between casting stages. In this way voids formed as the composition solidifies can be filled in a subsequent casting stage. However, this multi-stage approach to casting comes at the expense of productivity. The use of metal pins to define the detonator tunnels during casting also adds another step to the manufacturing process.
Against this background it would be desirable to adopt a different approach to the manufacture and use of cast boosters that does not suffer the operational and manufacturing issues noted above.